swarm-ui: agent roster page with per-agent config-PR status

New AgentsPage at /agents: fetches GET /api/agents (roster names) and
GET /api/config-prs (bulk config-PR status) and merges them into one
table, one row per agent. Reuses the existing Panel/Table/StatusChip/
RefreshIntervalPicker components exactly as HivesPage does — the roster
page and the config-PR panel turned out to be the same page rather than
two separate pieces of UI.

Adds a nav entry (green accent, the next unused base16 chromatic slot)
between hives and new agent.

frontend/packages/swarm-ui/src/shell/Shell.tsx

@@ -37,6 +37,7 @@ import './Shell.css';
 
 const NAV_ITEMS: { href: string; label: string; accent: string }[] = [
   { href: '/', label: 'hives', accent: 'var(--purple)' },
+  { href: '/agents', label: 'agents', accent: 'var(--green)' },
   { href: '/create-agent', label: 'new agent', accent: 'var(--cyan)' },
   { href: '/jobs', label: 'jobs', accent: 'var(--pink)' },
   { href: '/components', label: 'components', accent: 'var(--blue)' },